Telechelic and block copolymers are well known in the art and are employed for a variety of purposes. As block copolymers one of the more interesting varieties is of the configuration AB(BA).sub.n wherein n has a value of from about 1-10. One of the more interesting of block copolymers of this variety is that obtained when n is 1, or the block copolymer has a simple ABA configuration. Such block copolymers are often prepared to have a polystyrene-polybutadiene-polystyrene configuration; the A blocks may be homopolymers or copolymers. Usually the B block is an elastomer such as a butadiene polymer or an isoprene polymer segment. To obtain block copolymers of maximum uniformity, it is usually desirable to initiate polymerization employing a multifunctional lithium compound. In the case of an ABA block copolymer, a difunctional compound would be employed. In the case of an AB(BA).sub.3 block copolymer, a tetrafunctional initiator would be utilized. Generally, the multifunctional lithium containing initiators do not initiate as rapidly as smaller monofunctional compounds, and the polymerization product has a broader molecular weight range than is often desired. If it were possible to prepare block copolymers having very narrow molecular weight distribution which are often desirable for some applications, one could then blend such narrow molecular weight distribution block copolymers to obtain a product having almost any desired molecular weight distribution.
It would be desirable if there were available an improved process for the preparation of block copolymers of the AB(BA).sub.n variety.
It would also be desirable if there were available an improved process for the preparation of block copolymers of the AB(BA).sub.n variety which would result in product having a narrow molecular weight distribution.
It would also be desirable if there were available an initiator system which would provide such polymers having a narrow molecular weight distribution.
These benefits and other advantages in accordance with the present invention are achieved in a solution polymerization of at least one anionically polymerizable monomer wherein polymerization of the monomer is initiated in the presence of a multifunctional lithium containing soluble initiator compound to initiate polymerization and subsequently polymerize the anionically polymerizable monomer, the improvement which comprises employing as initiator system a mixture of the multifunctional lithium compound and a lithium alkoxide, the lithium alkoxide containing 2-16 carbon atoms and the equivalent ratio of alkoxide to multi-functional lithium compound being from about 0.05 to 2.
Also contemplated in the scope of the present invention is a polymerization initiator system comprising a hydrocarbon solvent, a multifunctional lithium containing polymerization initiator and the above described lithium alkoxide wherein the equivalent ratio of alkoxide to dilithium compound is from about 0.05 to 2.
Anionic polymerization is well known in the art, for example U.S. Pat. Nos. 4,431,777 and 4,427,837. Multifunctional initiators are well known and have been used in polymerization. Such initiators and their use are shown in the following U.S. Patents, the teachings of which are incorporated by reference thereto: U.S. Pat. Nos. 4,169,115; 4,172,100; 4,172,190; 4,427,837; 4,196,154; and 4,205,016.
Particularly desirable multifunctional lithium containing compounds are selected from the group consisting of the formula: ##STR1## wherein R.sub.1 is independently each occurrence hydrogen or an inert radical having from 0 to 16 carbon atoms;
R.sub.2 is a divalent organic radical having at least 6 carbon atoms, R.sub.2 having at least one aromatic ring and the aromatic ring being directly attached to a carbon which is attached to an aromatic ring of the above formula, with the further limitation R.sub.2 contains carbon and hydrocarbon, and optionally oxygen; oxygen when present is present only in the configuration of a diphenyl oxide; PA1 R.sub.3 is selected from the group consisting of alkyl, cycloalkyl, and aromatic radicals containing from 1 to 20 carbon atoms;
and especially preferred are initiating compounds of the Formula: ##STR2## wherein R.sub.1 and R.sub.3 are as previously defined.
Examples of the preceeding compounds and their use are set forth in U.S. Pat. Nos. 4,196,154 and 4,205,016.
In a preferred embodiment, R.sub.1 is selected from the group consisting of hydrogen, alkyl, cycloalkyl, alkoxy, aryl and mixtures thereof.
Polydiene monomers suitable for use in the practice of the present invention include 1,3-butadiene, isoprene and mixtures thereof.
Advantageously alkenyl aromatic monomers are also employed. By the term alkenyl aromatic monomer is meant a monomer of the formula: ##STR3## where n is an integer from 0 to 3, R.sub.4 is an alkyl radical containing up to 5 carbon atoms and R.sub.5 is hydrogen or methyl. Preferred alkenyl aromatic monomers include styrene, vinyltoluene, (all isomers alone or in admixture); particularly desirable is paravinyltoluene, or methyl styrene and the like.
Solvents useful for the practice of the present invention include hydrocarbons, both aliphatic and aromatic, as well as cycloaliphatic and include hexane, benzene, toluene, cyclohexane and in some instances a monomer can be employed also as a solvent, such as in the case of alpha-methylstyrene. In addition, a relatively polar solvent such as tetrahydrofuran, may also be utilized in combination with the initiator system of the invention in order to achieve varied microstructure in the resulting polydiene, as is previously known in the art.
Lithium alkoxides suitable for the practice of the present invention correspond to the formula LiOR wherein OR is an alkoxide moiety containing 2-6 carbon atoms and are readily prepared by the reaction of an alkyl lithium compound with an aliphatic alcohol having from 2 to 16 carbon atoms. Such alcohols may be monohydric or polyhydric and include ethanol, isopropyl alcohol, ethylene glycol and the like. Generally it is desirable to prepare the lithium alkoxide in solution, using an alkyl lithium compound as the lithium donor and a solvent compatible with the solvent used in the polymerization. The alkoxide may be prepared at any convenient temperature up to the polymerization temperature.
The multifunctional lithium compound and the lithium alkoxide may be combined at any time prior to initiation of polymerization. Generally, the equivalent ratio of alkoxide to the polylithium compound is from about 0.05 to 2 and advantageously from about 0.1 to 1.
Advantageously the lithium alkoxide can be made in the hydrocarbon solution of an organolithium compound, such as n-butyllithium by addition of the appropriate alcohol thereto.
The invention is further illustrated but not limited by the following examples.